In between those years, Avildsen directed 1976's Rocky and 1990's Rocky V and is credited with putting the film series on the map through his directorial vision. Interestingly, however, as the film's intro shows, many people don't know who directed the first Rocky — and most would guess writer and star Sylvester Stallone.

But from the ice skating scene, in which Adrian and Rocky go on their first date, to the choreography of Rocky and Apollo Creed (Carl Weathers) as they fight, Avildsen had his hand in just about everything — which he discusses in Underdog. Here, we've rounded up seven Rocky facts we picked up from the documentary:

As Rocky prepares to hit the ring for his big fight with Apollo Creed in the original Rocky, everyone on his fight team — Burgess Meredith's Mickey included — can be seen wearing pink sweaters with "Rocky" stitched in red lettering across the back.

As it turns out, those were pink by accident. The sweaters initially were white, but before filming, the wardrobe department washed them, and the color from the lettering ran. As a result, the rough, tough Rocky crew ended up in pink.

"We couldn't do it over again," Avildsen said of the error. "That's why they're pink. Nobody has asked, 'Why would he have a pink sweater?' "

Rocky features a tender, unforgettable scene between Rocky and Adrian that takes place at an empty ice skating rink, but initially, the scene was supposed to be just a coffee date. Viewing the original as "really boring," Avildsen suggested the skating date.

Because the film was shot on a relatively low budget, Avildsen said, crews sneaked into Philly and attempted to shoot exteriors for scenes with a nonunion crew. However, they gave up after Teamsters found out about the shoot and put pressure on the production to hire union crew members. Ultimately, the Rocky crew moved the ice skating rink scene production to Los Angeles to film the date.

Budget, too, resulted in the rink's emptiness. Producers said it would be too expensive to fill the frame with extras, so Avildsen suggested Rocky and Adrian go after the rink closed.

"Sylvester liked the idea and changed a couple of lines," Avildsen said. "It's so much sweeter and unique that way, and we didn't have to pay the extras."

3. Rocky wouldn’t have been Rocky without the Steadicam

Rocky featured a number of never-before-seen dynamic shots, many of which are featured in the film's beloved training montage — think Rocky running up the Art Museum steps or through the Italian Market. And though that might sound usual today, it wouldn't have been possible without the Steadicam — a camera invented by Philadelphia-born cinematographer Garrett Brown.

As Brown says in the documentary, Avildsen came across a demo reel of "30 impossible shots" that were made using the Steadicam — one of which was of a person running up the Art Museum steps.

Avildsen reached out to Brown and decided he wanted to use the camera rig on Rocky. And the rest, as they say, is history.

"We shot stuff that had never been seen before, some of which was due to the Steadicam, some of which was due to just the pure originality of the idea," Brown says. "Then it was that amazing rocket ride, and there's no rocket ride more fun than when it comes from the underdog status because you've got a lot more territory to cover on the way up."

4. Avildsen had never seen a boxing match, so they practiced fights ‘like a ballet’

When Avildsen was booked as the director of Rocky, he had never seen a live boxing match — ditto for composer Bill Conti, of the famed "Gonna Fly Now (Theme from Rocky)." So Avildsen approached the scene as something he was a little more familiar with: ballet.

Avildsen filmed rehearsal boxing matches between Stallone and Weathers and eventually suggested Stallone write out a fight in order to keep it from looking phony as the guys practiced. Stallone handed in "32 pages of lefts and rights," and the group went to work building the choreography. Avildsen provided his own form of encouragement. "I said, 'It doesn't look so good, guys. They're going to be looking at you, not me. And if you lost a couple of pounds, it wouldn't hurt.' "

Stallone, for his part, agreed. "Carl and I kind of became like a dance team," Stallone says. "We came within a hair's breadth of just knocking each other cold — and sometimes, we would hit each other."

5. Upon its release, some critics thought of Rocky as an “old-fashioned B-movie”

Film critic and historian Leonard Maltin explains in the doc that despite Rocky's theatrical success, some critics had a negative impression of the film, calling it an "old-fashioned B-movie." "In some ways, it resembled" that, Maltin says. "But it took the best elements of an old-fashioned B-movie and brought them up to date. I guess a modern film would have had more cynicism in it."

Maltin, as it turns out, isn't wrong. New York Times' Vincent Canby, for example, called the movie the "purest Hollywood make-believe of the 1930s" and said Stallone's portrayal of Rocky was little more than an "unconvincing actor imitating a lug." Time magazine's critic wrote that the script is "not merely improbable … it is preposterous." Variety said the film had "too obvious a feeling of really being manipulated and stroked."

It's now considered a classic.

By the way: Philadelphia Daily News film critic Joe Baltake and Inquirer film critic Desmond Ryan both enjoyed Rocky, but beloved Daily News boxing writer Stan Hochman did not: "Let's face it, the story is so flimsy, so trite, it would make you gag if not for the way Sylvester Stallone goes about telling the story," he wrote.

6. Avildsen said no to directing Rocky II for love

After its 1976 release, Rocky was considered a hit, but Avildsen passed on directing the followup, Rocky II, for romantic reasons.

As the director explains, he had begun dating Barra Grant, who worked as a screenwriter, and decided to make a movie called Slow Dancing in the Big City after Rocky's release. The movie went nowhere, and Avildsen and Grant didn't stay together. Avildsen regretted passing on the Rocky sequel. "In retrospect, I should have said yes, and we could have made Slow Dancing after that," Avildsen said. "But anyway, I wasn't that smart, and that didn't happen."

He may have passed on Rocky II, but Avildsen returned to the director's chair for Rocky V. Only it didn't end the way he wanted it — or the way Stallone wrote it.

Rocky was supposed to die with his head in Adrian's lap after his street fight with Tommy Gunn (Tommy Morrison). Adrian was set to give a speech encouraging people to keep Rocky's spirit alive to close the film. Calling it a "beautiful way to go out," Avildsen was game — but the studio wasn't.

Avildsen said, he received a call from a studio head telling him, "Rocky doesn't die."

The executive said, "These people don't die. James Bond doesn't die. Batman doesn't die. So, they don't die," Avildsen said. "He didn't die, but the movie died without him dying."

Actor Kevin Connolly, who appeared in Rocky V but who is better known for his starring role in Entourage, backed up Avildsen's version, saying the director had "signed up for something else."